Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried.The rates of absorption depend on a number of factors which are too variable to provide absolute dates.Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock.An object can be given an approximate date by dating the volcanic layers occurring above and below the object.Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.In special cases, bones can be compared by measuring chemicals within them.The number of tracks increases over time at a rate that depends on the uranium content.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.
Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.
Uranium is present in many different rocks and minerals, usually in the form of uranium-238.
This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.
The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
The technique can, however, provide the relative ages of bones from the same site.